1. Field of the Invention
The present invention relates to a silicon nitride circuit board and a semiconductor module which is useful as a semiconductor mounted circuit board, semiconductor package and the like.
2. Description of the Related Art
Recently, a ceramic circuit board prepared by bonding a metal plate (metal circuit plate) such as a copper plate to a ceramic plate is used as a substrate for mounting of a semiconductor module such as a power transistor module or semiconductor parts dealing relatively high electric power such as a switching current source module and the like.
As a bonding method of a ceramic substrate with a metal plate cut out in given shape in a production process for the ceramic circuit board as described above, a method using an active metal brazing material (active metal method) prepared by adding 1 to 10 wt. % of active metal such as Ti, Zr, Hf, Nb and the like to an Ag--Cu brazing material and the like, a so-called direct bonding method (DBC method: direct bonding copper method) in which a ceramic substrate is directly bonded to a copper plate using tough pitch electrolytic copper containing 100 to 1000 ppm oxygen or copper of which surface is oxidized in a thickness of 1 to 10 .mu.m as a metal plate, and the like are known.
For example, in the direct bonding method, a 0.3 to 0.5 mm thick copper circuit plate cut out in given shape is first contacted and placed on a 0.6 to 1.0 mm thick ceramic substrate composed of an aluminum oxide (Al.sub.2 O.sub.3) sintered material, aluminum nitride (AlN) sintered material and the like, and heated for formation of eutectic crystal liquid phase of Cu--Cu.sub.2 O on bonding interface, the surface of the ceramic substrate is wetted with this liquid phase, then, the liquid phase is cooled to be solidified for direct bonding of the ceramic substrate with the copper circuit plates. The ceramic circuit board obtained by using such a direct bonding method has advantages such as possibility of small size and high-density mounting, and the like, since bonding strength between the ceramic substrate with the copper circuit plates is high and it has simple structure in which a metallized layer or brazing material layer is not required. Further, the production process is intended to be shortened.
However, the above-described ceramic circuit board prepared by bonding a metal plate to a ceramic substrate by the direct bonding method, active metal method and the like has a problem that reliability is poor in heat history, since the metal plate has as large as 0.3 to 0.5 mm thickness for flowing large current. Namely, a ceramic substrate and a metal plate having extremely different heat expansion coefficients are bonded, heat stress derived from the above-described difference in heat expansion coefficient occurs by adding a cooling process and cooling cycle after the bonding. This stress exists in the form of remaining stress distribution of compression and tension on the ceramic substrate side near the bonding portion, and particularly, main stress of the remaining stress acts on ceramic portion adjacent to periphery end of the metal plates. This remaining stress causes cracking on the ceramic substrate, poor withstand voltage, releasing of the metal plate. Further, it exerts a reverse influence that strength of the ceramic substrate is lowered, even if no cracking occurs on the ceramic substrate.
With recent development of high density and highly integrated semiconductor element, miniaturization of a semiconductor module and electronic parts themselves is desired. Under these circumstances, miniaturization of the semiconductor mounted substrate per se is also required.
By the way, an aluminum nitride substrate has a high thermal conductivity and low heat expansion property as compared with other ceramic substrates. However, no aluminum nitride substrate having a sufficient mechanical strength has not been obtained yet. Therefore, when a slight pressure or impact is imparted to the circuit board in mounting process of the circuit board, the circuit board is easily destroyed, and production yield of a semiconductor apparatus is sometimes reduced steeply.
Therefore, in a conventional semiconductor module, it is essential factor to increase thickness of a ceramic substrate and integrate a reinforcing member with a circuit board. FIG. 4 is a cross-sectional view showing structure of such a conventional semiconductor module 101. The semiconductor module 101 shown in FIG. 4 has a large thickness, and is formed by bonding metal circuit plates 103 to a front surface of a ceramic substrate 102 made of an AlN sintered body having a high thermal conductivity, and at the same time by bonding a metal plate 104 as a backing copper plate to the rear surface of the ceramic substrate 102. Further, a semiconductor element 107 is mounted on a predetermined position of the metal circuit plate 103 by solder bonding, and an electrode portion of the metal circuit plate 103 and the semiconductor element 107 are electrically connected by a bonding wire 108.
The ceramic circuit board constructed as described above is integrated to the surface of a heat sink plate 105 made of, for example, copper by solder bonding, and a semiconductor module 101 as shown in FIG. 4 is formed. This semiconductor module 101 is fixed on an apparatus casing 109, a heat releasing fin or a mounting board by securing with an attaching screw 106.
However, the above-described conventional semiconductor module 101 or ceramic circuit board has problems that size becomes large when constructed as a semiconductor module and it is difficult to produce a small size module, and production cost for a ceramic substrate increases. In addition, since a heat sink plate which improves heat releasing property and prevents cracking of the ceramic substrate is required, and further, since it is necessary to use a ceramic substrate having a large thickness for enhancing resistance against cracking. Further, since thickness of the substrate increases, there is a problem that heat resistance increases and excellent heat releasing property as expected cannot be obtained even if an AlN substrate having a high thermal conductivity is used.
The present invention has been accomplished to cope with such problems, and a first object thereof is to provide a semiconductor module which does not require both a rear copper plate (rear metal plate) and a heat sink plate, of which structure is simple and can be miniaturized, and of which heat releasing property and heat cycle durability are improved.
On the other hand, with recent development of high density and highly integrated semiconductor element, miniaturization of a semiconductor module and electronic parts themselves is desired, and for high densely mounting, an enlargement of area for mounting part and circuit constituting part is desired.
However, in the conventional ceramic circuit board, since one main surface of a ceramic substrate is only used as a mounting surface, it is necessary to enlarge the ceramic substrate itself for increasing mounting part and circuit constituting part. When the ceramic substrate is enlarged, however, there may be posed problems that the ceramic substrate tends to bend in bonding process for bonding the copper circuit plates and the like. Further, simple enlargement of the ceramic substrate goes against the requirement for miniaturization of a mounting substrate and the electronic parts themselves.
As described above, in the conventional ceramic circuit board, the ceramic substrate should be enlarged for increasing the area of mounting part and the circuit constitutional part, and this invites poor bonding of the copper circuit plates and goes against requirement for miniaturization of the electronic parts. Further, with diversification of the semiconductor element, various properties are required for a semiconductor mounting substrate and semiconductor package, and it is desired to satisfy the various requirements.
On the other hand, high densely integration, high power performance and enlargement of a semiconductor element have been developed, and circuit board structure which can effectively dissipate heat generated from the semiconductor element outside of the system and circuit board structure having a high strength which can endure heat stress are technically required. Then, a silicon nitride (Si.sub.3 N.sub.4) substrate having higher strength than that of a conventional alumina (Al.sub.2 O.sub.3) substrate has been tried to be used as a ceramic substrate. However, since the Si.sub.3 N.sub.4 substrate has a remarkably lower thermal conductivity as compared with an aluminum nitride substrate and the like though it has excellent mechanical strength such as toughness and the like. Therefore, the Si.sub.3 N.sub.4 substrate is not practically used as a constitutional material of a circuit board for a semiconductor to which heat releasing property is particularly required.
On the other hand, the aluminum nitride (AlN) substrate has a higher thermal conductivity and lower heat expansion property as compared with other ceramic substrates, no aluminum nitride substrate having satisfactory mechanical strength has not been obtained yet.
Therefore, when slight pressure or impact is imparted to AlN substrate in mounting process of a circuit board, the circuit board is easily destroyed, and production yield of a semiconductor device is sometimes reduced steeply.
Therefore, in the conventional ceramic circuit board, it is essential requirement to increase thickness of the ceramic substrate. FIG. 7 is a cross-sectional view showing structure of such a conventional ceramic circuit board 201. The ceramic circuit board 201 as shown in FIG. 7 has a large thickness, and is formed by bonding two ceramic substrates 202a, 202b made of an AlN sintered body having a high thermal conductivity through a copper plate 204 as a metal circuit plate, by bonding a copper plate 203 to the upper surface of the ceramic substrate 202b, and by simultaneously bonding a metal plate 205 as a rear copper plate to the rear surface of the ceramic substrate 202a for integration. At a predetermined portion of the copper plate 203, a semiconductor element 207 is mounted by solder bonding, and an electrode portion of the copper plate 203 and the semiconductor element 207 are electrically connected by a bonding wire 208.
As described above, the above-described conventional ceramic (AlN) circuit board 201 has problems that the size is large when constructed as a semiconductor module and it is difficult to produce a small size module, and production cost for a ceramic substrate increases, since it is necessary to use ceramic substrates 202a, 202b each having a large thickness for improving the resistance against cracking. Further, since thickness of the substrate increases, there is a problem that heat resistance increases and excellent heat releasing property as expected cannot be obtained even if an AlN substrate having a high thermal conductivity is used.
The present invention has been accomplished to cope with such problems, and a second object thereof is to provide a silicon nitride circuit board which has an excellent heat releasing property, can enlarge area of mounting part and circuit constituting parts and can be miniaturized, and further, which has an improved heat cycle durability and excellent reliability.